1. Field of the Invention
The present invention relates to a sealed-by-resin type semiconductor device and a liquid crystal display module including the same. More particularly, the present invention relates to a COF (chip on flexible printed circuit) in which ICs and chips are implemented on a flexible substrate, and a liquid crystal display module including the same.
2. Description of the Related Art
Recently, there is a demand for smaller-sized, lighter, thinner elements for use in electronic devices such as personal digital assistants. One of such elements is a liquid crystal display (hereinafter referred to as LCD) module which is used as an output section of the electronic devices. Ease of incorporation into the devices is highly required.
Some LCD modules are created by the COF technique to meet the above-described demand. In such an LCD module, an IC for driving a liquid crystal (hereinafter referred to as an LC-driving IC) and other chips are mounted on a flexible substrate made of a polyimide film printed with a conductor pattern, and these elements are coupled to an LCD element via an anisotropic conductive film.
FIG. 5 is a plan view of such a COF LCD module. FIG. 6 is a side view of the COF LCD module.
A xe2x80x9csealing resinxe2x80x9d herein means a resin which fills between an LC-driving IC and a flexible substrate so as to protect a contact between the LC-driving IC and the flexible substrate.
As shown in FIG. 5, a COF LCD module 400 includes an LCD element 8 and a COF 300. The COF 300 includes a flexible substrate 9 on which an LC-driving IC 1 and a chip 10 are mounted.
The flexible substrate 9 is, for example, fabricated in the following way. A copper foil having a thickness of about 2 to 35 xcexcm is coated with a precursor of polyimide which is in turn cured. The resultant polyimide film substrate has a thickness of about 10 to 100 xcexcm. Such a fabricating method is called casting. The substrate is etched to obtain the desired conductor pattern. The substrate is coated with a polyimide resin or an epoxy resin, except for portions of the substrate 9 on which the LC-driving IC 1 and the chip 10 and contacts of the LCD element 8 with the LC-driving IC 1 and the chip 10. The conductor pattern on which a conductor is exposed is plated with Sn, Ni, Au, or the like. In this way, the flexible substrate 9 is fabricated.
As an alternative way to form the conductor pattern,an additive method may be employed. In this case, a sputtered copper is patterned and then thickened by plating.
The COF 300 is, for example, fabricated in the following way. The LC-driving IC 1 and the chip 10 are mounted on the conductor pattern of the flexible substrate 9. The LC-driving IC 1 is implemented by flip chip bonding.
The LC-driving IC 1 includes an Au bump (not shown) which is coupled with the conductor pattern. As a method of coupling the Au bump with the conductor pattern, for example, an Snxe2x80x94Au alloy coupling method, or a coupling method using an anisotropic conductive film may be adopted.
The Snxe2x80x94Au alloy coupling method is performed in the following way. The LC-driving IC 1 is provided on the flexible substrate 9 so that the Au bump of the LC-driving IC 1 contacts with the Sn-plated conductor pattern. The Au bump is coupled with the conductor pattern by heating and pressing the flexible substrate 9 from the rear side thereof (9A side). Subsequently, the LC-driving IC 1 is sealed by a sealing resin 4.
The coupling method using an anisotropic conductive film is performed in the following way. The anisotropic conductive film is interposed between the flexible substrate 9 and the LC-driving IC 1. In this situation, the flexible substrate 9 is heated and pressed from the rear side thereof (9A side) so that the Au bump is electrically coupled with the conductor pattern while the Au bump is fixed on the conductor pattern by the cured anisotropic conductive film.
Thereafter, the COF 300 fabricated as described above is conductive-coupled with the LCD element 8 using an anisotropic conductive film or the like, thereby obtaining the LCD module 400.
Recently, the pitch of the Au bump is becoming narrower in order to meet a demand for a higher resolution of liquid crystal display and a smaller area of the LC-driving IC 1. The Au bump is used as the segment output terminal of the LC-driving IC 1.
In order to improve the ease of incorporating the COF LCD module 400 into a device, the coupling strength between the LC-driving IC 1 and the flexible substrate 9 needs to be enhanced and the COF 300 on which the LC-driving IC 1 is mounted needs to be thinner.
The inventors fabricated and studied a prototype of the COF 300, in which the LC-driving IC 1 having an Au bump having a narrower pitch, is mounted on the flexible substrate 9 by the Snxe2x80x94Au alloy coupling.
As a result, when the pitch of the Au bump in the LC-driving IC 1 is about 70 xcexcm or less (a gap between the Au bumps is about 30 xcexcm or less), abnormalities in the liquid crystal display were encountered due to leakage between the Au bumps in a moisture-resistance reliability test in atmosphere having a humidity of about 95% at about 60xc2x0 C.
The abnormalities in liquid crystal display were investigated to find the causes. As a result, occurrence of migration was recognized between the Au bumps. It was found by elemental analysis that this migration was caused by Au.
In general, it is said that the Au migration is generated by an electric field being applied to a halogen and moisture.
According to one aspect of the present invention, a sealed-by-resin type semiconductor device includes a substrate; a lead provided on the substrate; and a semiconductor element provided on the lead by flip chip bonding. The semiconductor element includes a plurality of terminals connected to the lead; the sealed-by-resin type semiconductor device further includes a resin for protecting the plurality of terminals; and the resin has a sufficiently low elasticity modulus that occurrence of undesirable migration is suppressed.
In one embodiment of this invention, the elasticity modulus is substantially about 1 GPa or less.
In one embodiment of this invention, the elasticity modulus is substantially about 0.07 GPa or more and about 1 GPa or less.
In one embodiment of this invention, the resin includes a thermosetting resin, an epoxy resin, or a denatured polyimide resin.
In one embodiment of this invention, the resin includes an epoxy resin; and the epoxy resin includes a bisphenol type epoxy resin.
In one embodiment of this invention, the resin includes a denatured polyimide resin; and the denatured polyimide resin includes aromatic tetracarboxylic acid and aromatic diamine.
In one embodiment of this invention, the plurality of terminals include an Au bump.
In one embodiment of this invention, a pitch of the plurality of terminals is substantially about 70 xcexcm or less.
In one embodiment of this invention, the resin has a sufficiently high elasticity modulus that a coupling strength between the substrate and the semiconductor element is sufficient.
According to another aspect of the present invention, a liquid crystal display module includes a sealed-by-resin type semiconductor device including: a substrate; a lead provided on the substrate; and a semiconductor element provided on the lead by flip chip bonding, and a liquid crystal display element coupled with the substrate. The semiconductor element includes a plurality of terminals connected to the lead. The sealed-by-resin type semiconductor device further includes a resin for protecting the plurality of terminals. The resin has a sufficiently low elasticity modulus that occurrence of undesirable migration is suppressed.
In one embodiment of this invention, the liquid crystal display module further includes an anisotropic conductive film for coupling the substrate with the liquid crystal display element.
The inventors conducted an experiment in which an Au bump to which a halogen compound was attached was exposed to high temperature and high humidity. As a result, occurrence of migration was suppressed and a sufficient coupling strength was obtained between the LC-driving IC 1 and the flexible substrate 9 when the elasticity modulus of the sealing resin 4 sealing the LC-driving IC 1 was optimized. This led to achievement of the present invention.
According to the present invention, the electrical coupling reliability of the semiconductor elements and the flexible circuit substrate, such as heat-impact resistance, moisture resistance, and a coupling strength can be enhanced. An epoxy resin or a denatured polyimide resin is used as the sealing resin.
Thus, the invention described herein makes possible the advantages of (1) providing a sealed-by-resin type semiconductor device capable of suppressing migration between Au bumps, and an LCD module including the same and (2) providing a sealed-by-resin type semiconductor device having a sufficient coupling strength between an LC-driving IC and a flexible substrate.